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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-37417] Plans using $** wildcard indices can return duplicate results</title>
                <link>https://jira.mongodb.org/browse/SERVER-37417</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The query planner instructs an IXSCAN (or other index access stage) to deduplicate based on whether the index is multikey. &lt;tt&gt;$&amp;#42;&amp;#42;&lt;/tt&gt; indices, however, may contain multiple keys for a document in the absence of arrays. Consider the example of a collection which contains the document {a: {b: 1, c: 1}} with the index {&quot;$**&quot;: 1}. The index will contain the following keys, both referring to the same document:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;{$_path: &quot;a.b&quot;, &quot;a.b&quot;: 1}&lt;/li&gt;
	&lt;li&gt;{$_path: &quot;a.c&quot;, &quot;a.c&quot;: 1}&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;The planner, however, will generate an &lt;tt&gt;IndexEntry&lt;/tt&gt; which is not marked as multikey, since there are no array paths:&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/175f5e3c25ddba439b7d28254a4af5504aded0d8/src/mongo/db/query/planner_ixselect.cpp#L103-L110&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/175f5e3c25ddba439b7d28254a4af5504aded0d8/src/mongo/db/query/planner_ixselect.cpp#L103-L110&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;As a result, a &lt;tt&gt;$**&lt;/tt&gt; IXSCAN used to answer a query for which both of the index keys are in bounds will fail to deduplicate. The only known predicate for which this can happen is &lt;tt&gt;$exists&lt;/tt&gt;. See the repro steps below for an example query that returns the same document twice.&lt;/p&gt;</description>
                <environment></environment>
        <key id="612491">SERVER-37417</key>
            <summary>Plans using $** wildcard indices can return duplicate results</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="yuta.arai@mongodb.com">Yuta Arai</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Mon, 1 Oct 2018 22:03:27 +0000</created>
                <updated>Sun, 29 Oct 2023 22:27:44 +0000</updated>
                            <resolved>Mon, 15 Oct 2018 16:04:36 +0000</resolved>
                                                    <fixVersion>4.1.4</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="2032941" author="xgen-internal-githook" created="Mon, 15 Oct 2018 16:02:48 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;yarai&apos;, &apos;email&apos;: &apos;yuta.arai@10gen.com&apos;, &apos;username&apos;: &apos;yarai&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-37417&quot; title=&quot;Plans using $** wildcard indices can return duplicate results&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-37417&quot;&gt;&lt;del&gt;SERVER-37417&lt;/del&gt;&lt;/a&gt; Plans using $** wildcard indices can return duplicate results&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/e62512d50329877c84a7b2404c8c6158479efede&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/e62512d50329877c84a7b2404c8c6158479efede&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2020621" author="yuta.arai" created="Tue, 2 Oct 2018 15:05:41 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt; Our approach is pretty much what you said. When initializing the index scan stage in the execution level, we&apos;ll set the dedup flag to be true if there are more than one unique index bounds for wildcard indexes.&lt;/p&gt;</comment>
                            <comment id="2020425" author="david.storch" created="Tue, 2 Oct 2018 13:35:08 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=yuta.arai&quot; class=&quot;user-hover&quot; rel=&quot;yuta.arai&quot;&gt;yuta.arai&lt;/a&gt; &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=james.wahlin&quot; class=&quot;user-hover&quot; rel=&quot;james.wahlin&quot;&gt;james.wahlin&lt;/a&gt; what&apos;s your plan for solving this? I suppose we may need some special logic to instruct an index access stage to dedup if the bounds may contain keys with multiple &lt;tt&gt;$_path&lt;/tt&gt; values.&lt;/p&gt;</comment>
                    </comments>
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                    <name>Depends</name>
                                                                <inwardlinks description="is depended on by">
                                        <issuelink>
            <issuekey id="573403">SERVER-36198</issuekey>
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                            <customfieldvalue>&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;(function() {&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    &quot;use strict&quot;;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&amp;nbsp;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    db.c.drop();&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    assert.commandWorked(db.c.insert({a: {b: 1, c: 1}}));&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    assert.eq(1, db.c.find({a: {$exists: true}}).itcount());&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&amp;nbsp;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    assert.commandWorked(db.c.createIndex({&quot;$**&quot;: 1}));&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;    assert.eq(1, db.c.find({a: {$exists: true}}).itcount());&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;}());&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;/table&gt;
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